Field of the Invention
The present invention relates to a three-dimensional imaging apparatus using digital holography, such as a microscope device.
Description of the Related Art
In fields of medicine, biology and the like, as a method of observing a non-dyed biological object sample through a microscope device, C. D. Depeursinge, E. Cuche, P. Marquet, T. Colomb, P. Dahlgren, A. M. Marian, F. Montfort and P. J. Magistretti, “Digital Holography Applied to Microscopy” (Proceedings of SPIE, Vol. 4659, Practical Holography XVI and Holographic Materials VIII, pp. 30-34, Jun. 3, 2002, U.S.A.) discloses a method of utilizing digital holography.
The digital holography is a method of acquiring, through an image sensor, a digital signal corresponding to interference fringes generated by interference with a transmitted light transmitted through a sample and a reference beam and of calculating a wavefront of the transmitted light through a process performed by a processor 200 on the digital signal.
This method can quantitatively calculate a phase change of the wavefront caused by the sample, which enables observation of a non-dyed sample with high contrast.
On the other hand, biological object samples generally have a complicated three-dimensional structure, and observers such as medical doctors perform disease diagnosis on a basis of the three-dimensional solid structure.
Since a three-dimensional arrangement of cells is irregular, particularly in cancer, it is necessary to recognize this three-dimensional structure quickly in order to perform quick diagnosis.
Therefore, it is desired to acquire three-dimensional information of a sample and clearly provide the information to an observer.
Moreover, C. Fang-Yen, W. Choi, Y. Sung, C. J. Holbrow, R. R. Dasari, M. S. Feld, “Video-rate tomographic phase microscopy” (Journal of Biomedical Optics, Vol. 16(1), pp. 011005-1 to 011005-5, 14 Jan., 2011, U.S.A), which is hereinafter abbreviated as Literature A, discloses a three-dimensional imaging method of acquiring a three-dimensional refractive index distribution of a sample by performing a reconstruction process on multiple images of interference fringes produced through an image sensor; the images are produced by illuminating the sample with object beams whose incident angles to the sample are mutually different.
Interference fringes are normally generated by using light with high coherence, such as laser.
However, the light with high coherence generates a random phase distribution due to defects of an optical element through which this light passes and due to surface coarseness of the optical element, which changes an originally uniform illuminance distribution into a nonuniform illuminance distribution.
Such a nonuniform illuminance distribution is called speckle or speckle noise, which causes image degradation.
As a method of reducing the speckle noise, J. W. Goodman, “Speckle Phenomena In Optics” (Roberts and Company Publishers, Chapter 5, U.S.A.), which is hereinafter abbreviated as Literature B, discloses a method of averaging speckle by moving a diffusing plate.
However, the inventor of the present invention studying the three-dimensional imaging method disclosed in Literature A newly found a problem that noise is generated concentratively on a plane (focal plane) conjugate with the image sensor and this phenomenon affects the acquired refractive index distribution.
Although Literature A points out a similar problem to the above problem that a noise with a fixed pattern is generated in response to change of the incident angle of the object beam to the sample, it fails to describe about the noise generated concentratively on the focal plane.
On the focal plane there is much information on the sample which required by observers, and therefore image degradation on the focal plane is undesirable.
Moreover, the three-dimensional imaging method disclosed in Literature A requires acquisition of the multiple images of the interference fringes, which essentially increases time for data acquisition.
Furthermore, the method of simply equalizing the speckle disclosed in Literature B increases time for imaging, which is not desirable as a noise reduction method.